A natural resource such as oil or gas residing in a subterranean formation can be recovered by drilling a well into the formation. The subterranean formation is usually isolated from other formations using a technique known as cementing. In particular, a well bore is typically drilled down to the subterranean formation while circulating a drilling fluid through the well bore. After the drilling is terminated, a string of pipe (e.g. casing string) is run in the well bore. Primary cementing is then usually performed whereby a cement slurry is pumped down through the casing string and into the annulus between the casing string and the wall of the well bore or another casing string to allow the cement slurry to set into an impermeable cement column and thereby fill a portion of the annulus. Sealing the annulus typically occurs near the end of cementing operations after well completion fluids, such as spacer fluids and cements, are trapped in place to isolate these fluids within the annulus from areas outside the annulus. The annulus is conventionally sealed by closing a valve, energizing a seal, and the like.
After completion of the cementing operations, production of the oil or gas may commence. The oil and gas are produced at the surface after flowing through the casing string. As the oil and gas pass through the casing string, heat may be passed from such fluids through the casing string into the annulus. As a result, thermal expansion of the fluids in the annulus above the cement column causes an increase in pressure within the annulus also known as annular pressure buildup. Annular pressure buildup typically occurs because the annulus is sealed and its volume is fixed. Annular pressure buildup may cause damage to the well bore such as damage to the cement sheath, the casing, tubulars, and other equipment. In addition, annular pressure buildup makes proper casing design difficult if not impossible. Because the fluid pressures may be different in the annulus for each well bore, use of a standard casing design may not be practical. In order to control annular pressure buildup, conventional methods circulate gas into place during cementing operations. Because the gas is mobile, it is difficult to place the gas in the proper location and, at the same time, control the fluid pressure in the annulus. If, for example, the gas is placed too far below the top of the annulus, the rising gas will increase the pressure in the annulus.
Other techniques to control annular pressure buildup include pressure relieving/reducing methods, such as using syntactic foam wrapping on the casing string, placing nitrified spacer fluids above the cement column in the annulus, placing rupture disks in another, outer, casing string, designing “shortfalls” in the primary cementing operations, such as designing the top of the cement column in an annulus to be short of the previous casing shoe, and using hollow spheres. However, such techniques have drawbacks. For instance, the syntactic foam may cause flow restrictions during primary cementing operations. In addition, the syntactic foam may detach from the casing string and/or become damaged as the casing string is installed. Drawbacks with placing the nitrified spacer fluids include logistical difficulties (e.g., limited room for the accompanying surface equipment), pressure limitations on the well bore, and the typical high expenses related thereto. Further drawbacks with placing the nitrified spacer fluids include loss of returns when circulating the nitrified spacer into place and in situations wherein the geographic conditions provide difficulties in supplying the proper equipment for pumping the nitrified spacer. Additional drawbacks include failure of rupture disks that may prevent well bore operations from being able to proceed. Further drawbacks include the designed “shortfall,” which may not occur due to well bore fluids not being displaced as designed and cement channeling up to a casing shoe and trapping it. Moreover, problems with the hollow spheres include the spheres failing before placement in the annulus.